1. Field of the Invention
The present invention generally relates to the production of chalcopyrite compounds. In particular, it is concerned with an apparatus and a method for manufacturing an ABC.sub.2 chalcopyrite film having a wide application in the field of photovoltaic devices such as thin film-type solar cells.
2. Description of the Prior Art
In a polycrystalline thin film of the ABC.sub.2 chalcopyrite (wherein A represents Cu or Ag, B represents In, Ga or Al and C represents S, Se or Te), a thin film containing element A in excess has large crystal grains and is excellent in crystallinity of the crystal grains. Components of A.sub.2 C, for instance, Cu.sub.2 Se, Cu.sub.2 S and the like, however, are deposited on the boundary between the respective crystal grains or on the surface thereof, and thus, the thin film as it is cannot be applied to a device. An electrical characteristic of this thin film demonstrates a p-type conduction and its electric conductivity at room temperature is about 100 .OMEGA..sup.-1 cm.sup.-1. As mentioned above, although the thin film containing element A in excess is a polycrystalline thin film having an excellent crystallinity, it suffers a disadvantage that it holds an impurity phase on its boundary between the respective crystal grains or on its surface.
In spite of the above-mentioned disadvantage, it is reported that an excellent thin film available for the solar cells is formed based on such thin film (for instance, Proceedings of the 16th IEEE Photovoltaic Specialist Conference (1982), p.781). The disclosed manufacturing method is generally called "bilayer process". A scheme of this technology is shown in FIG. 10 and FIG. 11. The apparatus employed in the disclosed bilayer process comprises a vapor deposition apparatus including a vacuum chamber which accommodates cells 10, 11, and 12 each for supplying a flux of each of the elements A, B and C to a substrate 1, wherein the supplying amounts of the respective fluxes are controlled by the temperature of the respective cells. The substrate 1 is mounted on a substrate holder 4 and usually heated at 350.degree. C. or above.
The bilayer process using the apparatus shown in FIG. 10 proceeds in the following manner.
As shown in FIG. 11, by adjusting the amounts of the fluxes of the respective elements, an ABC.sub.2 chalcopyrite thin film layer 2 of a composition containing element A in excess is first produced (see, FIG. 11A). Subsequently, a second layer is deposited on the first produced layer by further elevating the temperature of the substrate by about 100.degree. C., and adjusting the amounts of the fluxes of the respective elements to the conditions under which an ABC.sub.2 chalcopyrite thin film layer of a composition containing element B in excess is produced (see, FIG. 11B). In this case, if the temperature of the substrate is sufficiently high, a thin film 2b in an intermediate state is converted into an ABC.sub.2 chalcopyrite thin film 3 of a homogeneous composition being approximate to a stoichiometric composition but containing element B in slight excess (see, FIG. 11C). The thin film layer produced in the above-mentioned manner has the large crystal grains and is free from the deposition of the impurity phase on the boundary between the respective crystal grains and on the surface, and thus becomes a thin film layer which can preferably be applied to the photovoltaic devices. The undesirable deposition of the impurity phase has frequently been observed with the ABC.sub.2 chalcopyrite of the composition containing element A in excess.
In order to produce a solar cell having a higher conversion efficiency, it is required to adjust a composition ratio of A/B at about 0.95-1.0 by accurately controlling the ratio. Although the above-mentioned bilayer process is one of the methods for producing an excellent ABC.sub.2 chalcopyrite thin film layer capable of being applied to the photovoltaic devices such as solar cells, it is difficult to realize an adequate composition ratio of A/B in a good reproducibility. The reason for this is that even if a condition for making the composition ratio of A/B is once established experimentally, the amounts of the fluxes of the respective elements sensitively reflect a difference in the degree of vacuum inside the vacuum chamber, a difference in the species of the gas remaining in the vacuum chamber and a delicate difference in the substrate temperature. Further, in this bilayer process, it is required to realize the following two conditions: one for producing an ABC.sub.2 thin film layer containing an excessive amount of element A and the other for producing an ABC.sub.2 thin film layer containing an excessive amount of element B, at the same time, by adjusting the amounts of the fluxes of the elements A and B. Therefore, this bilayer process has a disadvantage that it takes much time and labor for establishing these conditions that results in a poor throughput of the whole system.